The present invention relates to an optical disc drive capable of recording and/or reproducing data to/from various types of optical discs having different characteristics, such as a thickness of a protective layer and a data recording density.
There are a plurality of standards regarding the characteristics of the optical discs, including the thickness of the protective layer which covers a data recording surface of the optical disc and/or the data recording density. For example, the thickness of the protective layer of a CD (Compact Disc) or a CD-R (CD recordable) whose recording density is relatively low is 1.2 mm, while that of a DVD (Digital Versatile Disc) whose recording density is relatively high is 0.60 mm.
For recording and/or reproducing data to/from the DVD, since it has a relatively high data recording density, in order to make the size of a beam spot sufficiently small, a laser beam whose wavelength is in a range of approximately 635-660 nm is to be used. For the CD-R, in view of its reflection characteristics, a laser beam whose wavelength is approximately 780 nm is to be used.
The above-described optical discs may preferably be used in a single optical disc drive. In order to allow a single optical disc drive to use both the DVD and the CD-R, the disc drive is required to have at least two laser sources respectively emitting the laser beams as described above.
In the meantime, in view of downsizing of the disc drive, it is preferable that the optical system adopted in an optical head for the disc drive is as compact as possible. In particular, optical elements such as an objective lens is preferably used for both laser beams, which enables the laser sources (i.e., laser diodes) to be implemented in a single package and provided as a light source module. An example of such an optical disc drive is described in Japanese Patent Provisional Publication No. HEI 10-261240.
If the two laser diodes are incorporated in a single package, beam emitting points of the two laser diodes are aligned in a direction perpendicular to an optical axis of the objective lens. Therefore, at least one of the beam emitting points is located off the optical axis.
According to an embodiment described in the above-described publication, a semiconductor laser chip (wavelength: 660 nm) for a DVD, whose allowable aberration is relatively small, is positioned on an optical axis of a lens system including an objective lens, a collimating lens and the like. The other semiconductor laser chip (wavelength: 780 nm) for the CD is located off the optical axis of the lens system. Therefore, the optical disc drive described in the publication has a disadvantage such that aberration, in particular coma, is relatively large for the CD (or CD-R).
Another example of the optical disc drive is disclosed in Japanese Patent Provisional Publication No. HEI 10-261241. The optical disc drive disclosed in this publication includes the optical system similar to that disclosed in the aforementioned publication (i.e., HEI 10-261240), and further, a holographic optical element (HOE) is added. With this element (HOE), an optical path of the laser beam, whose beam emitting point is located off the optical axis, is deflected (bent) so that the beam is incident on an objective lens in a direction parallel to the optical axis, thereby suppressing the coma.
The HOE disclosed in the latter publication (i.e., HEI 10-261241) utilizes a zero order component of the beam for the DVD and a xe2x88x921st order diffraction component of the beam for the CD. Therefore, it is difficult to exhibit high diffraction efficiency for both wavelengths, and loss of light is relatively large. Further, since an additional element (i.e., HOE) having a special function as above is added, the manufacturing cost of the optical disc drive increases.
It is therefore an object of the invention to provide an improved optical disc drive that utilizes a light source module including at least two laser diodes emitting laser beams having different wavelengths, and a single objective optical system for both wavelengths. In this improved optical disc drive, aberration can be well suppressed with allowing recording and reproducing of data to/from a plurality of types of optical discs to be performed, without adding a special optical element such as the HOE.
For the above object, according to the invention, there is provided an optical disc drive capable of recording/reproducing data to/from an optical disc, the optical disc being either one of a first disc and a second disc, a protective layer of the first disc being thinner that that of the second disc, the optical disc drive including a first laser diode that emits a first laser beam having a first wavelength, a second laser diode that emits a second laser beam having a second wavelength, the second wavelength being longer than the first wavelength, an objective lens that converges the first laser beam on the first disc, and the second laser beam on the second disc, and a driving unit that holds and rotates the optical disc. In the optical disc drive constructed as above, an optical axis of the objective lens is inclined relative to a normal to the optical disc, a beam emitting point of the first laser diode is located at a first position, coma, which is caused when the first laser beam is converged on a data recording surface of the first disc, is minimized when the first laser beam is emitted from the first position. Further, a beam emitting point of the second laser diode is located at a second position which is different from the first position, and coma, which is caused when the second laser beam is converged on a data recording surface of the second disc, is minimized when the second laser beam is emitted from the second position.
With this configuration, whichever laser diode emits the laser beam, the coma is well suppressed, without employing an additional optical element such as HOE.
Optionally, the objective lens is configured such that coma is minimized for a hypothetical disc under a hypothetical condition, where the hypothetical disc is defined as an optical disc having a protective layer whose thickness is intermediate between that of the first disc and that of the second disc, and the hypothetical condition is defined as a condition where the optical axis of the objective lens coincides with the normal to the optical disc.
In this case, a first region may be defined on the objective lens, the first region providing a numerical aperture appropriate for converging the second laser beam on the second disc, and the objective lens preferably satisfies the following condition under the hypothetical condition where the optical axis of the objective lens coincides with the normal to the optical disc:
xe2x88x924.0 less than SC1/SC2 less than xe2x88x920.25, 
where, SC1 represents an offence SC against sine condition at the peripheral portion of the first region when the first laser beam is converged on the first disc,
SC2 represents an offence SC against sine condition at the peripheral portion of the first region when the second laser beam is converged on the second disc.
Further, the offence SC against the sine condition is defined by the formula below:
SC=nH1/(nxe2x80x2 sinUxe2x80x2)xe2x88x92f(1xe2x88x92m) 
where, n represents a refractive index on the beam incident side medium,
nxe2x80x2 represents a refractive index on the beam emerging side medium,
Uxe2x80x2 represents an angle of the emerging beam with respect to the optical axis,
m represents a paraxial magnification,
H1 represents a ray height on a principal plane, and
f represents a focal length.
Still optionally, the optical axis of the objective lens and the normal to the optical disc being included in a reference plane, the first position and the second position being located on opposite side with respect to a reference axis, and the reference axis is an optical axis of the objective lens under a hypothetical condition where the optical axis of the objective lens and the normal to the optical disc coincide with each other. Further, the reference plane is a plane including the reference axis and the first and second positions.
In this case, the first position and the second position may be arranged such that, by arranging the optical axis of the objective lens to be inclined with respect to the normal to the optical disc, the first laser beam is converged on a side where a distance between the objective lens and the optical disc increases, and the second laser beam is converged on a side where a distance between the objective lens and the optical disc decreases.
Furthermore, the objective lens is configured such that coma is minimized for a hypothetical disc under a hypothetical condition, the hypothetical disc being an optical disc having a protective layer whose thickness is intermediate between that of the first disc and that of the second disc, the hypothetical condition being a condition where the optical axis of the objective lens coincides with the normal to the optical disc.
Further optionally, the optical disc drive preferably includes a fine movement mechanism for driving the objective lens to move for focusing and tracking, the reference axis being inclined with respect to the normal to the optical disc, the objective lens being held by the fine movement mechanism such that the optical axis coincides with the reference axis.
In this case, the driving unit holds the optical disc such that the data recording surface of the optical disc extends in parallel with a bottom surface of a case of the optical disc drive.
In another case, the driving unit may hold the optical disc by inclining the data recording surface with respect to a bottom surface of a case of the optical disc drive so that the reference axis is perpendicular to a bottom surface of a case of the optical disc drive.
Alternatively, driving unit may hold the optical disc such that the data recording surface of the optical disc extends in parallel with a bottom surface of a case of the optical disc drive.